Field of the Invention
The invention relates to a device for generating comfort noise, and to a speech encoder and decoder including elements of such a device.
When speech signals are transmitted in network types transporting also data other than such signals, it is often useful to ensure that they do not occupy the whole pass-band and authorize the simultaneous passage of these other data, thus optimizing their bit-rates. Before transmission, a voice activity detector is provided with which the periods in which speech signals are present can be marked in input signals in which voice signals are mixed with noise and moments of silence.
If the presence of speech signals is detected, the subsequent speech encoder regularly transmits (in every frame) a stream of digital data which allows a distant receives to subsequently reconstitute these speech signals. If, in contrast, speech signals are no longer detected, encoded frames are no longer transmitted in the network so as to economize on their bit-rate. For the distant receiver, the signal samples are set at zero during these periods of speech absence. This solution is effective for bit-rate reduction, but may lead to erroneous unpleasant effects for the listener. Indeed, in the majority of cases, there is no total silence in the places where the conversation takes place, but rather, an ambient noise. If the input signal samples are set at zero at the moments of speech/silence transitions, the listener will have the impression of a discontinuity in the conversation, or even of a line cut-off.